CD40 is an approximately 50 kDa glycoprotein expressed on the surface of B cells, follicular dendritic cells, normal basal epithelium, and some carcinoma and melanoma derived cell lines (Paulie et al., 1985, Cancer Immunol. Immunother., 20:23-28; Clark and Ledbetter, 1986, Proc. Natl. Acad. Sci. 83:4494-4498; Ledbetter et al., 1987, J. Immunol. 138:788-794; Ledbetter et al., 1987, in "Leukocyte Typing III," McMichael, ed., Oxford U. Press pp. 432-435; Paulie et al., 1989, J. Immunol. 142:590-595; Young et al., 1989, Int. J. Cancer 43:786-794; Galay et al., 1992, J. Immunol. 149:775). Isolation of a human cDNA encoding CD40 showed that this protein is a type I membrane protein which is significantly related to the members of the nerve growth factor receptor family (Stamenkovic et al., 1989, EMBO J. 8:1403-1410).
The role of CD40 in B cell activation is well established. Crosslinking CD40 with anti-CD40 monoclonal antibodies (mAb) induces B cell aggregation via LFA-I (Gordon et al., 1988, J. Immunol. 140:1425-1430; Barrett et al., 1991, J. Immunol. 146:1722-1729), increases serine/threonine (Einfeld et al., 1988, EMBO J. 7:711-717) and tyrosine (Uckun et al., 1991, J. Biol. Chem. 266:17478-17485) phosphorylation of a number of intracellular substrates, and provides a "competency" signal which allows B cells to proliferate and undergo class switching when stimulated with the appropriate second signal. For example, anti-CD40 mAb can synergize with phorbol myristyl acetate (PMA; Gordon et al., 1987, Eur. J. Immunol. 17:1535-1538) or anti-CD20 Mab (Clark and Ledbetter, 1986, Proc. Natl. Acad. Sci. 83:4494-4498) to induce B cell proliferation, with IL-4 to induce B cell proliferation (Gordon et al., 1987, Eur. J. Immunol. 17:1535-1538; Rousset et al., 1991, J. Exp. Med. 173:705-710) and IgE secretion (Jabara et al., 1990, J. Exp. Med. 172:1861-1864; Rousset et al., 1991, J. Exp. Med. 173:705-710; Gascan et al., 1991, J. Immunol. 147:8-13; Zhang et al., 1991, J. Immunol. 146:1836-1842; Shapira et al. 1992, J. Exp. Med. 175:289-292) and with IL-10 and TGF-.beta. to induce IgA secretion by sIgD.sup.+ B cells (DeFrance et al., 1992, J. Exp. Med. 175:671-682). Also, there is evidence that CD40 delivered signals are involved in modulating cytokine production by activated B cells (Cairns et al., 1988, Eur. J. Immunol. 18:349-353; Clark and Shu, 1990, J. Immunol. 145:1400-1406).
Crosslinking of anti-CD40 mAb alone is not sufficient to induce B cell proliferation as demonstrated by the observation that anti-CD40 mAb immobilized on plastic in conjunction with IL-4 is unable to induce vigorous B cell proliferation (Banchereau et al., 1991, Science 251:70-72). However, anti-CD40 mAb immobilized on murine L cells transfected with an Fc receptor, CDw32, are able to induce B cell proliferation in the presence of IL-4 (Banchereau et al., 1991, Science 251:70-72), suggesting that a signal provided by the fibroblasts synergizes with the CD40 signal and IL-4 to drive B cell proliferation.